1. Field Of The Invention
The present invention relates to a multi-cast ABR service system and method for performing multi-cast ABR (Available Bit Rate) services in an ATM (Asynchronous Transfer Mode) network or other network.
2. Description Of The Related Art
This kind of multi-cast ABR service is used so that a plurality of receiving terminals could receive identical information at once after being broadcast from a sending terminal in an ATM or other network.
In the ABR (Available Bit Rate) service in the ATM network, a sending terminal recognizes the cell rate of the user cell capable of being transferred in a given time cycle and sends the cell to a receiving terminal by use of a cell called an RM (Resource Management), cell which is different from the user cell between the sending terminal and the receiving terminal. Thus, it is a service for making good use of the band area of a network while restraining the wasting rate of cell.
More specifically, a sending terminal periodically sends a Forward RM cell (hereinafter, abbreviated as an FRM cell) and a receiving terminal having received the FRM cell returns a Backward RM cell (hereinafter, abbreviated as a BRM cell). The rate capable of being transferred, which is called an ER (Explicit Rate), is set up in the RM cell (including the FRM cell and the BRM cell). The ER is rewritten in a receiving terminal and a switch, depending on the congestion in a network. In performing the multi-cast service by the ABR service using this rate control, it is necessary to put together the BRM cells sent from the respective receiving terminals into one RM cell in a switch which would be a branch point.
There are four conventional methods for putting together the BRM cells. Each method will be described hereinafter.
FIG. 4 is a block diagram showing the structure of a multi-cast ABR service system for realizing one of the above mentioned conventional methods. With reference to FIG. 4, a sending terminal 10 sends a Forward RM cell with some information and the transferable ER mounted thereon, to a switch 400. The switch 400 confirms the destination of the FRM cell according to the VPI and VCI included in the FRM cell sent from the sending terminal 10 and classifies each FRM cell. In the switch 400, ER rewriting units 410 and 430 respectively make a comparison between the ER within the RM cell and the ER set up beforehand and held by the ER rewriting units 410 and 430 and rewrite the ER within the RM cell to take the smaller ER. The switch 400 is provided with buffers 420 and 440 for use in sending the received RM cells. Upon receipt of an FRM cell through the switch 400, a given receiving terminal of the receiving terminals 41, 42, and 43 sends a Backward RM cell toward the sending terminal 10 having sent the FRM cell.
The operation of the multi-cast ABR service system shown in FIG. 4 will be described, this time. FIG. 5 is a flow chart for use in describing the operation of the system shown in FIG. 4. With reference to FIG. 5, the sending terminal 10 sends an FRM cell with the sending rate information [ER0] mounted thereon, to the receiving terminals 41 to 43 (Step 501). The FRM cell sent from the sending terminal 10 is broadcast through the switch 400 to all the receiving terminals 41 to 43 (Step 502). Thus, all the receiving terminals 41, 42, and 43 respectively receive the FRM cell (Step 503).
All the receiving terminals 41, 42, and 43, respectively having received the FRM cell, would try to return each BRM cell to the sending terminal 10. However, it is only the receiving terminal 41 that can return a BRM cell. This is because only a path from the receiving terminal 41 is extended to the sending terminal 10 and no path from the other receiving terminals 42 and 43 is extended to the sending terminal 10, in the structure of FIG. 4.
The receiving terminal 41 sends a BRM cell with the transferable ER [ER1] mounted thereon, to the sending terminal 10 (Step 504). The sending terminal 10 receives the BRM cell (Step 505) and the ER [ER1] within the BRM cell is used so as to perform a rate control of a network for making good use of a band area.
FIG. 6 is a block diagram showing the structure of a multi-cast ABR service system for realizing the other one of the conventional methods. With reference to FIG. 6, the sending terminal 10 sends an FRM cell with some information and the transferable ER mounted thereon to a switch 600. The switch 600 confirms the destination of the FRM cell according to the VPI and VCI included in the FRM cell having sent from the sending terminal 10 and classifies each FRM cell. The ER rewriting units 610 and 630 respectively make a comparison between the ER within the RM cell and the ER set up beforehand and held by the ER rewriting units 610 and 630 and rewrite the ER within the RM cell to take the smaller ER. The switch 600 is provided with buffers 620 and 640 for use in sending the received RM cells.
Upon receipt of the FRM cell through the switch 600, each of the receiving terminals 41, 42, and 43 sends a BRM cell toward the sending terminal 10 having sent the FRM cell. An ER comparison holding unit 650 provided in the switch 600 makes a comparison among the ERs in every received BRM cell and holds the smallest ER in an ER table 660. According to a timing control by a timer 680, the smallest ER being held is notified to a BRM cell generating unit 670. The BRM cell generating unit 670 generates a BRM cell according to the timing control by the timer 680. The timer 680 makes a notification toward the BRM cell generating unit 670 and the ER comparison holding unit 650 in every fixed period of time and controls the operation thereof.
The operation of the multi-cast ABR service system shown in FIG. 6 will be described this time. FIGS. 7 and 8 are flow charts for use in describing the operation of the system shown in FIG. 6. With reference to FIGS. 7 and 8, the sending terminal 10 sends an FRM cell with the sending rate information [ER0] mounted thereon, to the receiving terminals (Step 701). The FRM cell sent from the sending terminal 10 is broadcast through the switch 600 to all the receiving terminals 41 to 43. Thus, all the receiving terminals 41, 42, and 43 respectively receive the FRM cell. In the switch 600, the timer 680 starts counting (Step 702). Thereafter, the timer 680 continues counting to a predetermined value [X] and when the count value becomes [X], the timer 680 notifies it to the ER comparison holding unit 650 and the BRM cell generating unit 670.
All the receiving terminals 41, 42, and 43, each having received the FRM cell, return each BRM cell with the transferable ER for each receiving terminal 41, 42, and 43 mounted thereon, to the sending terminal 10 (Step 703). The ER [ER1] is mounted on the BRM cell as for the receiving terminal 41, the ER [ER5] is mounted on the BRM cell as for the receiving terminal 42, and the ER [ER3] is mounted on the BRM cell as for the receiving terminal 43. Hereinafter, the description will be made on assumption that the BRM cell 1 sent from the receiving terminal 41, the BRM cell 2 sent from the receiving terminal 42, and the BRM cell 3 sent from the receiving terminal 43 would reach the switch 600 in this order.
The BRM cells sent from all the receiving terminals 41, 42, and 43 are sent to the ER comparison holding unit 650, the ERs mounted on the respective BRM cells are compared, and the smallest ER is held in the ER table 660. The ER comparison holding unit 650 makes a comparison of the ERs of the BRM cells successively received and holds the smaller ER selectively (refer to Steps 706, 707, 714, 715, 722, and 723).
As illustrated in FIG. 6, the ER of the BRM cell sent from the receiving terminal 41 is defined as [ER1], the ER of the BRM cell sent from the receiving terminal 42 is defined as [ER5], the ER of the BRM cell sent from the receiving terminal 43 is defined as [ER3], and the size of each ER is defined as ER5&lt;ER1&lt;ER3. In this case, the smallest ER for the sending terminal 10 is obtained as the following procedure.
a) Min {initial table value (ER), ER1}=ER1
b) Min {ER1, ER5}=ER5
c) Min {ER5, ER3}=ERS
If the timer 680 makes a notification toward the ER comparison holding unit 650 and the BRM cell generating unit 670 at the timing of the procedure 3, the smallest ER becomes [ER5] (Step 715), the BRM cell which has been generated in the BRM cell generating unit 670 is sent to the sending terminal 10 with the ER [ER5] mounted thereon (Steps 725 to 727). The value for the sending terminal 10 in the ER table 660 is rewritten into the initial value.
Further, in the other multi-cast ABR service system of the conventional methods, all the receiving terminals having received the FRM cells would return each BRM cell. A switch judges the sending source (receiving terminal) of the respective BRM cell every time of receiving a BRM cell. By comparison of the ERs of the respective BRM cells, the smaller ER is held. After receiving the BRM cells from all the receiving terminals, the smallest ER left in the last comparison is mounted on the BRM cell received last, which is sent to the sending terminal.
Further, the other multi-cast ABR service system of the conventional methods is provided with a switch counter for counting every time of receiving BRM cells. The counter counts the number of all the receiving terminals. The switch makes a comparison of the ERs of the received BRM cells until the counter counts to the number of all the receiving terminals, and holds the smaller ER in every comparison. After the count number becomes the number of all the receiving terminals, the smallest ER left through the last comparison is mounted on the BRM cell received last, which is sent to the sending terminal.
The above conventional techniques, however, have the following problems.
The first conventional technique shown in FIGS. 4 and 5, provided with only one predetermined receiving terminal that can send a BRM cell, is defective in that the transferable ERs of the other receiving terminals cannot be reflected in a sending terminal. This is why a path for sending a BRM cell from a receiving terminal to a sending terminal is extended only from one receiving terminal and no path is extended from the other receiving terminals.
The second conventional technique shown in FIGS. 6 to 8 is defective in that the structure of a switch becomes complicated because the switch needs a function of generating a BRM cell. The smallest ER must be added to the generated BRM cell and it must be sent to a sending terminal.
The third conventional technique is incapable of sending a BRM cell through a switch to a sending terminal unless the switch receives all the BRM cells sent from the respective receiving terminals. Therefore, if the switch fails to receive even one BRM cell of all, the sending terminal cannot receive the BRM in reply to the sent FRM cell, thereby causing deterioration in reliability of a system.
In the fourth conventional technique, a switch sends a BRM cell to a sending terminal only in case of receiving the same number of BRM cells as the number of the all receiving terminals. Therefore, a counter for counting the number of received BRM cells must be provided in the switch, which makes the structure of the switch complicated. Further, if the number of the received BRM cells doesn't meet the number of all the receiving terminals, a sending terminal cannot receive the BRM corresponding to the FRM cell sent by the own self, thereby causing deterioration in reliability of a system.